It has long been known that additive noise dither with optimum amplitude and proper statistics can improve the spurious free dynamic range (SFDR) of a digital-to-analog converter (DAC) or an analog-to-digital converter (ADC). Use of dithering, however, to achieve precision DC bias for the purpose of mixer LO leakage suppression has not been attempted.
Current generation synthetic aperture radars (SAR) developed at Sandia National Laboratories generally employ a receiver/exciter construct that comprise fundamental components of a high performance, wide bandwidth radar receiver/exciter subsystem. As shown in FIG. 1, a digital waveform synthesizer (DWS) 110, operating at a clock frequency of 1 GHz, with a single-ended output 105, is capable of directly synthesizing chirp waveforms with a maximum bandwidth of approximately 375 MHz, after filtering 140. An RF up-converter 120 with a ×8 bandwidth multiplication chain can realize a system with a maximum RF bandwidth capability of 3 GHz. Broad RF bandwidth operation (fine range resolution) can be emphasized with this configuration, so stretch processing (de-ramp mixing) 170 is generally used to compress the RF bandwidth to a much narrower IF bandwidth. A high-speed, dual accumulator, DWS 110 is typically the source of transmit and receiver LO linear FM (chirp) waveform. At the receiving end generally is a high-performance, high dynamic range receiver/digitizer 160.
The single ended DWS 110 of FIG. 1 has fundamental limitations, which can be summarized as follows:                The theoretical maximum synthesized bandwidth is ½ of the DWS clock frequency.        The single ended DWS output is filtered, translated, and bandwidth multiplied to obtain the necessary LO frequency.        
What is needed in the art are improved SAR systems that can overcome the foregoing limitation associated with radar dynamic range.